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Re^3: grep and dereferencing an array of arrays

by McA (Priest)
on Sep 06, 2013 at 19:06 UTC ( #1052755=note: print w/replies, xml ) Need Help??

in reply to Re^2: grep and dereferencing an array of arrays
in thread grep and dereferencing an array of arrays

Can you show me Dumper($qablock);

Update: I don't know what you want to achieve with the first code block. Do you really just want the last matching result?


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Re^4: grep and dereferencing an array of arrays
by oakb (Scribe) on Sep 06, 2013 at 19:35 UTC
    The program is a self-grading multiple-choice testing environment. The teacher can enter an arbitrary number of questions, with four possible answers to each question. Only one of the answers may be correct. The program randomizes the questions and answers, so a student will never get the same test twice (and can't cheat using a scoring key). This example is a simple spelling test. Three of the answers are wrong ('W') and one is right ('R'):
    ========== $VAR1 = [ [ 'a', 'W', 'interupt' ], [ 'b', 'R', 'interrupt' ], [ 'c', 'W', 'innterupt' ], [ 'd', 'W', 'intterupt' ] ]; ==========
    Since the answers have been randomized, the program has to match the entered answer letter ('a..d') to the 'W' or 'R' that is the second element of one of the elements of the $qablock. Only then can the actual answer be recorded for later scoring (since the answer 'b' has no lasting significance).

    The reason that I removed the square brackets from the grep is because the element of $qablock that the grep is assigning should already be an anonymous array reference. But then I still had to use the brackets for the assignment in the foreach, so I tried it with brackets as well. Basically, I've been flailing in the hope of getting lucky....

      Ok, that puts some light on your problem. This may be your solution:

      #!/usr/bin/env perl use strict; use warnings; use 5.010; use Data::Dumper; my $choices = [ [ 'a', 'W', 'interupt', ], [ 'b', 'R', 'interrupt', ], [ 'c', 'W', 'innterupt', ], [ 'd', 'W', 'intterupt', ] ]; my $answer = 'd'; # As we know that the answer matches only one choice # and we have exactly one matching answer. my $chosen_answer = (grep { $_->[0] eq $answer } @$choices)[0]; say Dumper($chosen_answer);


        Or, maybe more readable for some:
        my ($chosen_answer) = grep { $_->[0] eq $answer } @$choices;
        لսႽ ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
      I am not sure what the objective is. However, a couple of possibilities come to mind... map{} can return a conditional thing...use () to return "nothing". See below.
      #!/usr/bin/perl -w use strict; my $VAR1 = [ [ 'a', 'W', 'interupt' ], [ 'b', 'R', 'interrupt' ], [ 'c', 'W', 'innterupt' ], [ 'd', 'W', 'intterupt' ] ]; print "Number of Questions: ", scalar @$VAR1, "\n"; #@$VAR1 produces a list of references to the arrays #The map selects the 2nd one of each array, index [1] #the grep in a scalar context counts them up print "Number right: ", scalar( grep{/r/i}map{@$_[1]}@$VAR1 ), "\n"; print "Number wrong: ", scalar( grep{/w/i}map{@$_[1]}@$VAR1 ), "\n"; #Maybe the question is which one is the "right" answer, a,b,c,d? print "\n"; print "correct answer is: ", map{@$_[1] =~ /r/i ? @$_[0]:()}@$VAR1; __END__ Number of Questions: 4 Number right: 1 Number wrong: 3 correct answer is: b Process completed successfully

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[Corion]: You'll have to look somewhere esoteric for that. Maybe some tied variable or special dualvar can also trigger that. But it's certainly not a common occurrence
[Corion]: And on 5.20, the following also outputs no find:perl -wle 'for my $x ("\x{2000}".."\ x{1fffff}") { if( $x && ! length $x ) { warn qq(<$x>); warn length $x; die } }'
[Corion]: (this time on Unix)
[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.
[Corion]: Aaah - you should be able to do this with overload, but I would hit somebody really hard if they constructed objects that are true but the empty string, and you not knowing about the domain knowledge where this makes sense
[Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
[Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
[Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context
[hippo]: That's equal parts clever and horrendous.

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